There has been a desire for an instrument to monitor constituents in an animal or human organ non-invasively. A particular example is monitoring of oxygen level in the brain which is particularly important, for example during surgery, where a significant number of patients come out of the anesthesia with various degrees of and sometimes permanent brain function deficiency. It is believed that in a significant portion of such cases, lack of sufficient oxygen to the brain is the cause of such deficiencies. Thus, the ability to accurately monitor oxygen level in the brain directly, rather than through indirect methods such as a pulse oximeter placed on another portion of the body, would have obvious advantages including non-invasiveness, immediate and timely results, and relative simplicity. Techniques to achieve such monitoring have involved passing near-infrared radiation through a cranium and analyzing the modified output radiation.
One known method is to pass radiation having several discrete wavelengths from laser diodes equal in number to the number of constituents to be measured, for example two wavelengths for oxygenated and deoxygenated hemoglobin. The radiation is modulated with radio frequency. The output modified by the brain is used to calculate changes in amplitude and phase which lead to determination of absorption coefficients at the different wavelengths. Simultaneous equations with these coefficients determine concentrations of the constituents of interest and the oxygen saturation which is the percentage of oxygenated to total hemoglobin.
Another method is to utilize continuous-wave radiation, in which output from a detector on a cranium is spectrally analyzed to yield oxygen saturation. Although a full spectrum is used, the analysis is based on modeling with either a small number of wavelengths or a few known constituents such as the oxy and deoxy hemoglobin and water.
Any such monitoring encounters difficulties resulting from the biological complexities of an organ such as a brain, compared with spectrometric instrumentation that ordinarily analyzes fluids that are readily probed, contained or flowing in a tube suitable for the instrument. Geometries of different subjects vary considerably and variations occur even within an individual. Further, tissues are not uniform. The radiation is scattered so that a path is not well defined. Signal to noise ratios for infrared radiation through solid material are generally low. Current methods for monitoring of craniums depend on theoretical or mathematical models that may be oversimplified or inaccurate. Thus there is a need for better accuracy and reproducibility.
Consequently, an object of the invention is to provide a novel method and means for monitoring constituents in an animal organ non-invasively, particularly oxygenated and deoxygenated hemoglobin in a brain.
The foregoing and other objects are achieved by a method or an apparatus for monitoring one or more selected constituents in an animal organ, with a spectrometric instrument that includes a source of an input beam of infrared radiation having a substantially full spectrum in a spectral range that includes absorbance wavelengths of the selected constituents, and a spectral detector receptive of such radiation to generate representative signal data. The instrument may be, advantageously, an infrared Fourier transform spectrometric instrument. The input beam is directed into an animal organ such that the radiation is attenuated by constituents of the organ including the selected constituents. The spectral detector is positioned so as to be receptive of the attenuated radiation from an exit site from the organ so as to generate signal data representative of spectral distribution of the attenuated radiation. Spectral intensities are calculated over the spectral range from the signal data. Concentrations of the selected constituents are computed from the spectral intensities and from a predetermined statistical correlation model relating such concentrations and spectral intensities. Advantageously the radiation is passed through a cranium such that the radiation is attenuated by brain constituents, particularly oxygenated hemoglobin and de-oxygenated hemoglobin, utilizing a spectral range from about 700 nm to about 1100 nm. Saturation level of oxygenated hemoglobin may be computed relative to a total of the oxygenated hemoglobin and de-oxygenated hemoglobin, whereby the saturation level is independent of path length of the radiation to the spectral detector.
To predetermine the correlation model, the foregoing procedures may be effected for a plurality of organs of a same type with each organ having established concentrations of the selected constituents, and the correlation model is statistically determined from the concentrations and corresponding intensities.
Intensities are preferably converted to absorbances, the concentrations being computed from the correlation model and the absorbances. For more accurate computation of concentrations, path length of the radiation is ascertained in the organ between the input site and the exit site, and each absorbance for each spectral increment is divided by the path length to effect normalized absorbances, the concentrations being computed from the correlation model and the normalized absorbances. To ascertain path length, a further beam of input discrete radiation comprises at least one discrete wavelength component in the spectral range, each wavelength component being modulated with a radio frequency signal. The further beam is directed into the organ at the input site such that the discrete radiation is modified by the organ. A radiation detector is positioned to be receptive of the modified radiation from the exit site so as to generate corresponding detector signals. A phase shift is determined between the radio frequency signal and the corresponding signals, and thereby between the input discrete radiation and the modified radiation for each discrete wavelength. From each phase shift, correspondingly at least one effective path length of the discrete radiation is calculated in the organ between the input site and the exit site. From each effective path length, a spectral path length is computed for each spectral increment in the spectral range. Each absorbance for each spectral increment is divided by the increment path length for that spectral increment to effect the normalized absorbances.